Allow spaces after attribute names.

Using `AttributeSyntax.ToFullString()` was breaking detection of Refit
attributes if there was a space between the attribute name and the
bracket, because the "full" string includes trivia.

Replacing with `AttributeSyntax.ToString()` fixed that problem and
didn't break any other tests so I think it's good.

Fixes #73.
Merge pull request #74 from bennor/allow-spaces-after-attribute-names

Allow spaces after attribute names.
disable unassigned warnings for fields in the PreserveAttribute class

Resolves issue #76 whereby visual studio would spew warnings
that RefitInternalGenerated.PreserveAttribute.X is never
assigned.
Merge pull request #77 from ghuntley/issue76

disable unassigned warnings for fields in the PreserveAttribute class