AddRequestParameter double encodes parameter value

Describe the bug
AddRequestParameter encodes the already-encoded value when other parameter is encoded.

There is an remote endpoint which has two request parameters(Say, a and b) which each may need to be url-encoded.

Within my route, I'm intending putting the b parameter's value.

Per, #2717, I configured the b parameter's value as url-encoded.

When sending a with an ascii value, it works.

When sending a with an url-encoded value, the b parameter's value is double-encoded.

# application.yaml

spring:
  cloud:
    gateway:
      routes:
        filters:
          - AddRequestParameter=b, %2F

CLIENT                  GATEWAY
--------------------------------------------------
a=%2F                   a=%2F&b=%2F   OK
a=a                     a=a&b=%252F   NOT OK

Start at the AddRequestParameter filter entry point and reproduce the issue with the application.yaml route configuration shown here, testing both encoded and ASCII values for parameter a. Trace how the filter handles the configured b value; done means an already URL-encoded b value is not double-encoded when a is ASCII.