Bug 903123: review comment: Stream entity body to MessageDigest.

I implemented this in HawkAuthHeaderProvider rather than provide a
generic method to digest a BaseResource's entity body because
AuthHeaderProvider's are lower level than the BaseResource that
invokes them.  I am considering factoring out this Hawk code into a
small Java library, and such a tie would make that harder.
Bug 903123: review comment: Prefer StringBuilder to StringBuffer.
Bug 903123: review comment: Prefer direct returns rather than named variable returns.
Bug 903123: review comment: Factor out extra string escaping utilities.

I've opened https://github.com/hueniverse/hawk/issues/89 to inquire
about the different escaping.

I didn't see testing the helpers adding significant value.