query: honor Encoder on interface fields

[c-tonneslan]

reflectValue checks sv.Type().Implements(encoderType) on the static field type. When a field is declared as interface{}, that type is the empty interface, which doesn't implement Encoder, so a value stored there never gets its EncodeValues method called and falls back to default formatting (e.g. v=[a b] instead of v.0=a&v.1=b).

Unwrapping the interface to its concrete value before the check fixes it, and lets the value be formatted by its dynamic type in general. Added an interface{}-field case to TestValues_CustomEncodingSlice; it produced v=[a b] before the change.

Fixes #121

[willnorris]

looks good to me, thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7af347e) to head (121e633).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #187   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          165       140   -25     
=========================================
- Hits           165       140   -25     

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