[BUG] Failed to add required=false

Description:
There is not required=false in in RequestPart from generated API source code.

We call API without file (possibly by specification of API), then we receive:
2019-01-31 13:18:52.506 WARN 13343 --- [nio-2004-exec-5] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.web.multipart.support.MissingServletRequestPartException: Required request part 'file' is not present]

When replace RequestPart("file") to @RequestPart(value="file", required=false), I do not receive this error, how previous expected. In this way, we add to pom.xml:
<replacement> <token>RequestPart\(\"file\"\)</token> <value>RequestPart\(value=\"file\"\, required=false\)</value> </replacement>

openapi-generator version: 3.3.4

OpenAPI declaration file content:

` summary: 'Create a marker'
  description: 'Create a new Marker with a optional media, that can be audio or image'
  requestBody:
    content:
      multipart/form-data:
        schema:
          type: object
          required:
            - marker
          properties:
            marker:
              type: object
              $ref: '#/components/schemas/Marker'
            file:
              type: string
              format: binary

`
link: https://github.com/lztcf/bug-openapi3/blob/master/src/main/resources/api.yaml

Command line used for generation: mvn generate-sources
Part of generated code:

    default ResponseEntity<Marker> markerPost(@ApiParam(value = "", required=true, defaultValue="null") @RequestParam(value="marker", required=true)  Marker marker,@ApiParam(value = "file detail") @Valid @RequestPart("file") MultipartFile file) {

Steps to reproduce:

I made a little project that reproduce the bug: https://github.com/lztcf/bug-openapi3

Related issues/PRs:

Like described in https://github.com/OpenAPITools/openapi-generator/issues/1646, we replaced RequestParam to RequestPart: @RequestParam(value="marker", required=true) to @RequestPart(value="marker", required=true).

Start with the OpenAPI declaration's multipart/form-data requestBody and the generated Spring method containing @RequestPart("file"). Run mvn generate-sources against the linked reproducer and compare the generated annotation with the optional file property in the specification. Done means optional multipart files generate required=false without breaking the required marker parameter.