rc: allow JSON parameters to simplify command line usage

If the parameter being passed is an object then it can be passed as a
JSON string rather than using the `--json` flag which simplifies the
command line.

rclone rc operations/list fs=/tmp remote=test opt='{"showHash": true}'

Rather than

rclone rc operations/list --json '{"fs": "/tmp", "remote": "test", "opt": {"showHash": true}}'

docs/content/rc.md

@@ -165,6 +165,8 @@ $ rclone rc rc/noop param1=one param2=two
 Run `rclone rc` on its own to see the help for the installed remote
 control commands.
 
+## JSON input
+
 `rclone rc` also supports a `--json` flag which can be used to send
 more complicated input parameters.
 
@@ -184,6 +186,22 @@ $ rclone rc --json '{ "p1": [1,"2",null,4], "p2": { "a":1, "b":2 } }' rc/noop
 }
 ```
 
+If the parameter being passed is an object then it can be passed as a
+JSON string rather than using the `--json` flag which simplifies the
+command line.
+
+```
+rclone rc operations/list fs=/tmp remote=test opt='{"showHash": true}'
+```
+
+Rather than
+
+```
+rclone rc operations/list --json '{"fs": "/tmp", "remote": "test", "opt": {"showHash": true}}'
+```
+
+
+
 ## Special parameters
 
 The rc interface supports some special parameters which apply to

fs/rc/params.go

@@ -219,6 +219,13 @@ func (p Params) GetStruct(key string, out interface{}) error {
 	}
 	err = Reshape(out, value)
 	if err != nil {
+		if valueStr, ok := value.(string); ok {
+			// try to unmarshal as JSON if string
+			err = json.Unmarshal([]byte(valueStr), out)
+			if err == nil {
+				return nil
+			}
+		}
 		return ErrParamInvalid{errors.Wrapf(err, "key %q", key)}
 	}
 	return nil

fs/rc/params_test.go

@@ -304,6 +304,20 @@ func TestParamsGetStruct(t *testing.T) {
 	assert.Equal(t, true, IsErrParamInvalid(e3), e3.Error())
 }
 
+func TestParamsGetStructString(t *testing.T) {
+	in := Params{
+		"struct": `{"String": "one", "Float": 4.2}`,
+	}
+	var out struct {
+		String string
+		Float  float64
+	}
+	e1 := in.GetStruct("struct", &out)
+	assert.NoError(t, e1)
+	assert.Equal(t, "one", out.String)
+	assert.Equal(t, 4.2, out.Float)
+}
+
 func TestParamsGetStructMissingOK(t *testing.T) {
 	in := Params{
 		"struct": Params{