ServletExternalContextImpl.encodeRedirectURL() doesn't handle existing query parameters correctly

ServletExternalContextImpl.encodeRedirectURL() doesn't work correctly if the base URL already contains encoded query parameters. In this case the resulting URL contains the query parameter encoded twice, which completely breaks them.

Take a look at this example:

externalContext.encodeRedirectURL("/test?p1=a+b", null);

Result ----> /test?p1=a%2Bb

Another example are encoded ampersand:

externalContext.encodeRedirectURL("/test?p1=a%26b", null);

Result ----> /test?p1=a%2526b

The root cause of the problem seems to be that ServletExternalContextImpl.encodeURL() doesn't decode the query parameters while parsing the base URL.