Decouple HttpRequest from GenericUrl to allow generic url building

Right now HttpRequest and HttpRequestFactory require GenericUrl to
eventually construct a URL string when callers eventually call
HttpRequest#execute(); HttpRequest @ 863: 

-- 8< --
String urlString = url.build();

-- 8< --

Reduce coupling between GenericUrl and HttpRequest / HttpRequestFactory
from URL string building to allow others to provide their own URL
building logic.  GenericUrl, for most people, will do what they want;
especially if they conform to UriTemplate's RFC 6570.

My rationale is that GenericUrl is designed to not allow easy extending
to avoid change in the parsing behaviour of query parameter construction
due to initial design and parsing constraints.

It is reasonable that one might like to design a similar class to
GenericUrl such that the convenience of query parameter construction is
maintained by Genericdata, but the actual representation of query
parameters may be different.  For example, MyGenericUrl would construct
query parameter strings to appear as the following:

-- 8< --
class A extends MyGenericUrl {
    @Key List<String> list;
    @key Map<String, Object> map;
}

Map<String, Object> map = new HashMap<String, Object)>();
map.put("key1", "val1");
map.put("key2", "val2");
map.put("key3", "val3");

A a = new A();
a.list = Arrays.asList("val1", "val2");
a.map = map;

a.build();

-- 8< --

Might output something like:
?list[]=val1&list[]=val2&map[key1]=val1&map[key2]=val2&map[key3]=val3


I understand this would be a binary incompatible change.  Is there a
better way to accomplish something like this, considering the current
design of GenericUrl?