Conform to RFC6902 replacement semantics.

A "replace" patch operation referencing a key that does not exist in the
target object are currently being accepted; ultimately becoming "add"
operations on the target object. This is incorrect per the
specification:

From https://tools.ietf.org/html/rfc6902#section-4.3 section about
"replace":

    The target location MUST exist for the operation to be successful.

This corrects the behavior by returning an error in this situation.

patch.go

@@ -384,13 +384,17 @@ func (d *partialDoc) add(key string, val *lazyNode) error {
 }
 
 func (d *partialDoc) get(key string) (*lazyNode, error) {
-	return (*d)[key], nil
+	v, ok := (*d)[key]
+	if !ok {
+		return v, errors.Wrapf(ErrMissing, "unable to get nonexistent key: %s", key)
+	}
+	return v, nil
 }
 
 func (d *partialDoc) remove(key string) error {
 	_, ok := (*d)[key]
 	if !ok {
-		return errors.Wrapf(ErrMissing, "Unable to remove nonexistent key: %s", key)
+		return errors.Wrapf(ErrMissing, "unable to remove nonexistent key: %s", key)
 	}
 
 	delete(*d, key)
@@ -612,7 +616,7 @@ func (p Patch) test(doc *container, op Operation) error {
 	}
 
 	val, err := con.get(key)
-	if err != nil {
+	if err != nil && errors.Cause(err) != ErrMissing {
 		return errors.Wrapf(err, "error in test for path: '%s'", path)
 	}
 

patch_test.go

@@ -332,6 +332,10 @@ var BadCases = []BadCase{
 		`{ "foo": [ "all", "grass", "cows", "eat" ] }`,
 		`[ { "op": "move", "from": "/foo/1", "path": "/foo/4" } ]`,
 	},
+	{
+		`{ "baz": "qux" }`,
+		`[ { "op": "replace", "path": "/foo", "value": "bar" } ]`,
+	},
 }
 
 // This is not thread safe, so we cannot run patch tests in parallel.