Description of problem:
Evolution does not strip the leading space on the value of a custom header
before comparing it for filtering.

Version-Release number of selected component (if applicable):
evolution-(none):2.10.3-2.fc7.i386

Steps to Reproduce:
Create a filter on a "Specific header", specifying that it "starts with" the
text. Notice how it doesn't filter. Add a single space before the text and
notice how it now filters.