Allow '=' in values.

GetString reported error ("missing =") for values that contained equal
sign. For example, the following base64-encoded value failed to parse:

 hash=Qtp+wEkYRc0tslUOpW0iYsUjAHj4Uv0VYZu1CQZgsmQ=
Add test for values with equals sign.
Remove temporary files after tests.
Merge pull request #1 from dchest/equals

Allow '=' in values.