Make PathParams public

[lavoiesl]

This PR makes the PathParams property public, same as QueryParam, FormData, Header, etc.

I don't really see the value in having it private and it prevents me from adding a middleware to capture all the existing parameters.

Example:

func InjectContextMiddleware(service string) resty.RequestMiddleware {
	return func(c *resty.Client, req *resty.Request) error {
		ctx := req.Context()
		ctx = logger.WithFields(ctx, map[string]interface{}{
			"http_service":         service,
			"http_service_host":    c.HostURL,
			"http_request_method":  req.Method,
			"http_request_uri":     req.URL, // URLs should have fairly low cardinality. Users should use PathParams and QueryParams for variable parts
			"http_request_attempt": req.Attempt,
			"http_request_retry":   req.Attempt > 1,
		})

		for k, v := range req.QueryParam {
			if len(v) > 1 {
				ctx = logger.WithField(ctx, "http_request_query_"+k, v)
			} else {
				// most common scenario, only one value; inline the value instead of serializing an array
				ctx = logger.WithField(ctx, "http_request_query_"+k, v[0])
			}
		}

		// Currently not possible
		for k, v := range req.PathParams {
			ctx = logger.WithField(ctx, "http_request_path_"+k, v)
		}

		req.SetContext(ctx)
		return nil
	}
}

[jeevatkm]

@lavoiesl Thanks for the PR and improvements to Resty.
Currently, Travis CI build is not helping me to validate this PR, do you mind executing tests locally and attach it to the PR?
I will include this PR in v2.7.0.
Of course, I will work on #479 to sort out for the long term.

[lavoiesl]

Sure thing 👍

$ go version
go version go1.11.13 darwin/amd64
$ go test ./...
ok  	github.com/go-resty/resty/v2	170.098s

[jeevatkm]

@lavoiesl Thanks for the test execution info.