Java RestTemplate generator should format correctly java8 dates in query parameters

Hi,

Having the given spec :

openapi: 3.0.0
info:
  version: "1.0"
  title: API
servers:
  - description: Mock
    url: ***
paths:
  /catalog/products:
    get:
      summary: finds products
      operationId: findProducts
      tags: 
        - catalog
      parameters:
        - in: query
          name: modifiedSince
          description: filter products modified since the given value
          required: false
          schema:
            type: string
            format: date-time
      responses:
        '200':
          description: products
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Product'
        '400':
          description: bad input parameter
components:
  schemas:
    Product:
      type: object
      required:
        - id
      properties:
        id:
          type: int

I'm using the swagger-codegen-maven-plugin with the given configuration :

<configuration>
	<inputSpec>v1/openapi.yaml</inputSpec>
	<language>java</language>
	<invokerPackage>${project.groupId}.api.v1.client.invoker</invokerPackage>
	<apiPackage>${project.groupId}.api.v1.client</apiPackage>
	<modelPackage>${project.groupId}.api.v1.model</modelPackage>
	<library>resttemplate</library>
	<generateApiTests>false</generateApiTests>
	<generateApiDocumentation>false</generateApiDocumentation>
	<configOptions>
		<dateLibrary>java8</dateLibrary>
		<java8>true</java8>
	</configOptions>
</configuration>

This generate the following client API :

public class CatalogApi {

    public List<Product> findProducts(OffsetDateTime modifiedSince) throws RestClientException {
        ...
    }

}

However, when calling this method with an OfffsetDateTime, like this :

catalogApi.findProducts(OffsetDateTime.now());

It generates the following HTTP request :

GET /catalog/products?modifiedSince=2019-02-05T14:57:54.928+01:00

The server answer with a 400 Bad request.

I suspect that the "+" sign is interpreted as the "space" character from the server in this case and it fails to deserialize the value correctly in the appropriate OffsetDateTime type...

Looking at the template "/src/main/resources/handlebars/Java/libraries/resttemplate/ApiClient.mustache" method "parameterToString" there is a special case to handle formatting of java.util.Date but nothing to handle java 8 dates, such as OffsetDateTime...

Start in src/main/resources/handlebars/Java/libraries/resttemplate/ApiClient.mustache, at the parameterToString method and its existing java.util.Date handling. Reproduce the generated request with an OffsetDateTime query parameter, then verify that the resulting date-time value is correctly formatted for the server instead of treating the plus sign as a space.