Remove Request#process_payload, which hasn't actually been used in ages.
Also rewrite the tests that called it to actually test code that is in
use, even though they are bad unit tests.
Remove HashMapForTesting, which was never used.
The previous boundary used a random number up to 1,000,000 as the
boundary. This is extremely low entropy compared to what most major
browsers use (20 bits). Instead, use a convention more like WebKit.
RestClient previously would be completely unable to upload a file
generated like so, containing `--0--` through `--1000000--`:
File.open('foo.txt', 'w') {|f|
1_000_000.times {|i| f.write("--#{i}--\n") }}
Instead, the boundary now looks like this, with about 95 bits of entropy:
`----RubyFormBoundaryFg2MqiXXQlT5RkUF`
* Put unit and integration specs into separate subdirectories
* Consolidate all requires of 'webmock/rspec' into spec_helper.rb
* Use WebMock.{disable!,enable!} for specs calling live server
