Change the maximum line length limit using Gmail API

Question

I am trying to send emails by Gmail API with Python. But when I check the received email,I find that it seems that each row is automatically broken at 78th chars. The screen shot can be seen as below:

It seems to be caused by RFC 2045. But can I reset some parameters to make it longer for each line?

My code can be seen as below:

import base64
from email.mime.audio import MIMEAudio
from email.mime.base import MIMEBase
from email.mime.image import MIMEImage
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
import mimetypes
import os

#from __future__ import print_function
import httplib2
import os

from apiclient import discovery
import oauth2client
from oauth2client import client
from oauth2client import tools

from apiclient import errors

SCOPES = 'https://www.googleapis.com/auth/gmail.compose'
CLIENT_SECRET_FILE = 'client_secret.json'
APPLICATION_NAME = 'send survey about assistance tool for Stack Overflow'

try:
        import argparse
        flags = argparse.ArgumentParser(parents=[tools.argparser]).parse_args()
except ImportError:
        flags = None

def SendMessage(service, user_id, message):
    """Send an email message.

    Args:
        service: Authorized Gmail API service instance.
        user_id: User's email address. The special value "me"
        can be used to indicate the authenticated user.
        message: Message to be sent.

    Returns:
        Sent Message.
    """
    try:
        message = (service.users().messages().send(userId=user_id, body=message)
                             .execute())
        print 'Message Id: %s' % message['id']
        return message
    except errors.HttpError, error:
        print 'An error occurred: %s' % error


def CreateMessage(sender, to, subject, message_text):
    """Create a message for an email.

    Args:
        sender: Email address of the sender.
        to: Email address of the receiver.
        subject: The subject of the email message.
        message_text: The text of the email message.

    Returns:
        An object containing a base64url encoded email object.
    """
    message = MIMEText(message_text)
    message['to'] = to
    message['from'] = sender
    message['subject'] = subject
    return {'raw': base64.urlsafe_b64encode(message.as_string())}


def get_credentials():
        """Gets valid user credentials from storage.

        If nothing has been stored, or if the stored credentials are invalid,
        the OAuth2 flow is completed to obtain the new credentials.

        Returns:
                Credentials, the obtained credential.
        """
        home_dir = os.path.expanduser('~')
        credential_dir = os.path.join(home_dir, '.credentials')
        if not os.path.exists(credential_dir):
                os.makedirs(credential_dir)
        credential_path = os.path.join(credential_dir,
                                                                     'sendEmail.json')

        store = oauth2client.file.Storage(credential_path)
        credentials = store.get()
        if not credentials or credentials.invalid:
                flow = client.flow_from_clientsecrets(CLIENT_SECRET_FILE, SCOPES)
                flow.user_agent = APPLICATION_NAME
                if flags:
                        credentials = tools.run_flow(flow, store, flags)
                else: # Needed only for compatibility with Python 2.6
                        credentials = tools.run(flow, store)
                print('Storing credentials to ' + credential_path)
        return credentials
if __name__ == "__main__":
    credentials = get_credentials()
    http = credentials.authorize(httplib2.Http())
    service = discovery.build('gmail', 'v1', http=http)
    SendMessage(service, "me", CreateMessage("sender@gmail", "receiver@gmail.com", "Testing sending emails", "I want all my one sentence to be in each line and it will breaked by my specified line breakers.\n But it seems the line will break at 78 chars and can I reset that parameters to make it longer?"))

Any suggestions will be welcomed and thanks in advance.