Python 3: Use HTTPMessage.get() rather than HTTPMessage.getheader()

In Python 2, get() is a synonym for getheader(), which does not exist in Python
3.

Closes-Bug: #1267987
Change-Id: I6989a6ea733e0a186e9e68a61aaa8c3e43a198a6
Merge "Python 3: Use HTTPMessage.get() rather than HTTPMessage.getheader()"