query: honor Encoder on interface fields

reflectValue checked sv.Type().Implements(encoderType) against the static
field type, so a value stored in an interface{} field never used its
EncodeValues method and fell back to default formatting. Unwrap the interface
first so the concrete type's Encoder is seen.

Fixes #121

Signed-off-by: Charlie Tonneslan <cst0520@gmail.com>

query/encode.go

diff --git a/query/encode.go b/query/encode.go
index c936954..3b058bb 100644
--- a/query/encode.go
+++ b/query/encode.go
@@ -186,6 +186,11 @@
 			continue
 		}
 
+		// unwrap interface values so the concrete type's Encoder is used
+		if sv.Kind() == reflect.Interface && !sv.IsNil() {
+			sv = sv.Elem()
+		}
+
 		if sv.Type().Implements(encoderType) {
 			// if sv is a nil pointer and the custom encoder is defined on a non-pointer
 			// method receiver, set sv to the zero value of the underlying type

query/encode_test.go

diff --git a/query/encode_test.go b/query/encode_test.go
index a94b44d..241e4f1 100644
--- a/query/encode_test.go
+++ b/query/encode_test.go
@@ -477,6 +477,14 @@
 			}{(*customEncodedStrings)(&[]string{"a", "b"})},
 			url.Values{"v.0": {"a"}, "v.1": {"b"}},
 		},
+
+		// custom encoded type held in an interface field
+		{
+			struct {
+				V interface{} `url:"v"`
+			}{customEncodedStrings{"a", "b"}},
+			url.Values{"v.0": {"a"}, "v.1": {"b"}},
+		},
 	}
 
 	for _, tt := range tests {