Use equal? to compare form_input to rack.input

Using equal? provides consistent results of equality between 1.8, 1.9,
and 2.0 when comparing Tempfile objects.

In 1.8, == will change the position of the Tempfile.
In 1.9+, == compares Tempfiles correctly.

In 1.8, eql? compares Tempfiles correctly
In 1.9+, t.eql?(t) always returns false
Merge pull request #756 from filipegiusti/fix-large-posts

Rack 1.5: Use equal? to compare form_input to rack.input