Stop relying on the message text of ZeroDivisionError in tests.

The test for broken_repr relied on the message text of
ZeroDivisionError, which is different on pypy (just "integer
division", not "integer division or modulo"). This causes a test
failure. Fix this by explicitly raising an exception with a message we
provide.
Fix the 'processes' comment which is a bit misleading.

It says "number of processes to spawn", but what it does is enable
the SocketServer.ForkingMixIn which 'handles each request in a new process'.

Its value is not the number of processes but the maximum number of processes
that the ForkinMixIn will allow at the same time.
Adding HTTPException subclasses for HTTP statuses 428, 429, and 431,
which were recently added in RFC 6585.
Remove a duplicate line
missed HTTP_STATUS_CODES
Change the example in parse_options_header

This seems to be a more typical usage.
contrib.atom: Fix copy and paste error in docstring.
Merge pull request #227 from maix/master

 contrib.atom: Fix copy and paste error in docstring
Merge pull request #208 from SimonSapin/patch-4

Change the example in parse_options_header
Merge pull request #202 from SimonSapin/patch-1

Remove a duplicate line
Fixed an error with openssl serving
Fix an error when using 'adhoc' as ssl_context
Merge pull request #198 from typingduck/master

Comment fix
Update docs for FileStorage
Remove docs reference to import_module
Merge pull request #200 from jessaustin/master

Add HTTPExceptions for 428, 429, and 431
Merge pull request #240 from vreon/patch-1

Remove docs reference to import_module
Fix for issue #239
Added '+' to always_safe
Added the example url from the test to the testsuite.
Support for db number in redis cache
Added db in Redis connection