UriComponentsBuilder supports query without value

Fix UriComponentsBuilder to support query parameters that do not
include a value without losing '='. The following styles are now
supported:

    http://example.com/foo?bar=baz
    http://example.com/foo?bar=
    http://example.com/foo?bar

Issue: SPR-10215
Align UriComponents.toUri() with toUriString()

Update HierarchicalUriComponents.toUri() to only prepend a missing '/'
when the scheme, user info, host or port are specified. This makes
the toUri() method behave in the same way as .toUriString() and allows
relative URIs to be created.

Issue: SPR-10231