Fix default value for a header

This code specified None as the default value, but it would raise an
exception if that was ever returned.  An empty string causes the code to
work as intended.

This came up while I was writing a unit test for another bug fix.

Change-Id: I658bb8a9b5124f281988fb60b645182ea0ccf99f
Related-bug: #1231524
Fix misused assertTrue in unit tests

Refactored unit tests to use assertEqual instead of assertTrue
where needed.

Closes-Bug: #1226374

Change-Id: I5f3b582e19f3461a04b5df05960095779ec8aa1a
Merge "Fix default value for a header"