feat: allow use of strict field names

This change allows configuring a Decoder to not match folded field names by using the UseStrictNames function. It should be noted this is not exhaustively tested at this stage.

decode.go

@@ -217,6 +217,7 @@ type decodeState struct {
 	savedError            error
 	useNumber             bool
 	disallowUnknownFields bool
+	strict                bool
 }
 
 // readIndex returns the position of the last byte read.
@@ -703,7 +704,7 @@ func (d *decodeState) object(v reflect.Value) error {
 			if i, ok := fields.nameIndex[string(key)]; ok {
 				// Found an exact name match.
 				f = &fields.list[i]
-			} else {
+			} else if !d.strict {
 				// Fall back to the expensive case-insensitive
 				// linear search.
 				for i := range fields.list {

stream.go

@@ -36,6 +36,9 @@ func NewDecoder(r io.Reader) *Decoder {
 // Number instead of as a float64.
 func (dec *Decoder) UseNumber() { dec.d.useNumber = true }
 
+// UseStrictNames causes the Decoder to unmarshal only to exact field names.
+func (dec *Decoder) UseStrictNames() { dec.d.strict = true }
+
 // DisallowUnknownFields causes the Decoder to return an error when the destination
 // is a struct and the input contains object keys which do not match any
 // non-ignored, exported fields in the destination.