Fix remove circular require

See the warning issued by HTTPI v2.2.5 in its context:
https://travis-ci.org/the4dpatrick/possible-email/jobs/32727448

The warning was also issued by HTTPI v2.2.7.
Minor fix ambiguous syntax

Both files did issue "`*' interpreted as argument prefix" warnings
in HTTPI 2.2.7.
Minor fix method redefinition

HTTPI::Response#attachments was defined twice, which caused
a warning to be issued by HTTI 2.2.7.
Minor fix assigned but unused variables

Both assignments caused a warning to be issed by HTTPI 2.2.7.
Merge pull request #132 from acidlabs/fix-circular-require

Fix a few warnings, among which a circular require