Previously, ActionDispatch::IntegrationTest would always set
CONTENT_TYPE on the request whether or not the value being set was a
string or nil. However, Rack SPEC requires that if CONTENT_TYPE is set,
it must be a string.
Since the request_encoder can return nil for #content_type (and the
IdentityEncoder always will), IntegrationTest must check before it sets
the CONTENT_TYPE value.
A Rack::Lint test has been added to prevent regressions. Additionally,
it will make changes needed for Rack 3 more obvious when the time comes.
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