Flask: non-JSON input mappers, send_*_resp content types, 400 on bad bodies (#40)

[thorwhalen]

Fixes the first three items of #40; the fourth (Bottle-only configs) is split to #42.

• Raw, binary and form input mappers read the body/files through helpers that work for Bottle and Flask requests (duck-typed on get_data).
• Under Flask a mapper that declares a content_type and returns a str/bytes body (send_json_resp, send_raw_resp, send_binary_resp) now answers that content type (flask_response_mapper, applied by framework_defaults).
• Empty, non-JSON or non-object bodies, a missing __fields form part and a wrong request content type raise InputError (400) for both frameworks instead of a 500 (previously RuntimeError / werkzeug text).
• Tests: py2http/tests/test_flask_requests.py. Dependents gate (extrude): pass.

🤖 Generated with Claude Code

[thorwhalen]

Adversarial review (fresh Opus): no Bottle regressions; 1 confirmed regression (Flask rejecting BOM/UTF-16 JSON that master accepted) and case-sensitive content type, both fixed with tests; remaining minor gaps recorded on #42.