refactor: minor tweaks to /drip implementation

[mccutchen]

A handful of drive-by tweaks to the /drip implementation:

• Update docs to clarify intended uses (a helpful reminder for myself, if nothing else)
• Ensure all bad requests use standard JSON error responses
• Switch from binary to text content type

This also includes a small fix to the internal testing library, dropping the assert.RoughDuration() helper in favor of an updated generic assert.RoughlyEqual() implementation that works with the time.Duration type.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.13%. Comparing base (24529f4) to head (5574709).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
- Coverage   95.13%   95.13%   -0.01%     
==========================================
  Files          10       10              
  Lines        1726     1725       -1     
==========================================
- Hits         1642     1641       -1     
  Misses         48       48              
  Partials       36       36              

Files with missing lines	Coverage Δ
httpbin/handlers.go	99.21% <100.00%> (-0.01%)	⬇️