Bad http.url when Rails raises exceptions

[delner]

While working on improvements for our exception handling support in Rails, I discovered that our http.url was sometimes being given bad URLs after Rails handled exceptions.

At core of this issue, the Rack tracer needs a source of truth for the http.url value. Looking at the env hash, it has a few possible options which to derive this value:

• PATH_INFO: Must be present on Rack requests. Starts as relative URL value, e.g. /article/1?foo=bar. It's often overwritten by Rails whenever an exception occurs, to the new path its rendering e.g. /500. At end of the trace, this value isn't very useful.
• REQUEST_URI: Equal to relative URL value, e.g. /article/1?foo=bar. Appears to be provided depending on the web server, may not always be present (for example, when using IIS instead of WEBrick/Puma.)
• ORIGINAL_FULLPATH: Rails only. Equal to relative URL value, e.g. /article/1?foo=bar. Unlike PATH_INFO, it doesn't get mutated.

The original logic was previously env['REQUEST_URI'] || env['PATH_INFO']. This logic works under the most common of cases, but provides a /500 value when REQUEST_URI isn't present, and an exception is raised in Rails.

This pull request proposes the new value should simply be the original value of PATH_INFO, before the rest of the Rack stack runs. This should universally address all cases.