Transform (trim) on nested objects via definitions is not working

Hello!

I have found interesting behavior of transform keyword, which is probably a bug.
In short: `"transform": [ "trim" ]` is not working on nested objects if specified via definitions.

The schema:
```
{
"$schema": "http://json-schema.org/draft-07/schema#",
"definitions": {
"trimmedString": {
"type": "string",
"transform": ["trim"]
}
},
"type": "object",
"properties": {
"a": {
"$ref": "#/definitions/trimmedString"
},
"nested": {
"type": "object",
"properties": {
"b": {
"$ref": "#/definitions/trimmedString"
},
"c": {
"type": "string",
"transform": ["trim"]
}
}
}
}
}
```

Input data:
```
{
"a": " x ",
"nested": {
"b": " y ",
"c": " z "
}
}
```

Expected results: all strings are trimmed.

Actual results: string `b` in nested objects is not trimmed
```
{
"a": "x",
"nested": {
"b": " y ",
"c": "z"
}
}
```

My versions:
```
"ajv": "^7.0.3",
"ajv-formats": "^1.5.1",
"ajv-keywords": "^4.0.0",
```

Thanks!