Contact Forms: Token replacement only in POST.

Only add the token replacement filter when processing form POST requests. Fixes #829.
Contact Forms: Consistent subject filter calls.

Add the field name to value array to the other subject filter application for consistency.
Contact Forms: Tests add token replacement hook.

Tests now account for the new placement of the subject token replacement hook addition.
Merge pull request #830 from Automattic/fix/contact-forms-PHP-errors

Contact Forms: Token replacement only in POST. Fixes #829