rewrite test client redirect handling

[davidism]

• EnvironBuilder.from_environ creates a builder from an existing environ. The previous code was passing EnvironBuilder(environ), but that doesn't mean anything to __init__.
• Redirect environ is initialized from initial environ and passed to open, rather than passing only some args to open.
• Script root and path are correctly distinguished. The root is preserved from the initial environ, stripped from the location, or cleared if the location is not under the root. Fixes Test client incorrectly follows redirects with a non-root base_url #382
• HEAD method is not changed to GET.
• Headers are passed to new request for all codes. There's nothing for or against passing them in the RFC, but Chrome, Firefox, and Requests all do this. Fixes Test Client does not keep Authorization in the request headers when following redirects #1151
• Body and relevant headers are dropped except for 307 and 308. Previously the client would never pass on the body, it would only preserve the method for 307.
• 1EnvironBuilder.get_environ` sets content type and length headers in addition to WSGI keys when detecting from data.
• Better error message when disallowing domain redirects. Distinguishes between "subdomains aren't enabled" and "can't go to external host."
• Test client recognizes 308 code.
• Add 308 to http codes.

[davidism]

Another change in #1403, intermediate redirect response bodies are exhausted even when buffered=False.