Erase elements
This function removes an element from the container.
All iterators that are equal to `pos` or come after are invalidated.
url u( "?first=John&last=Doe" );
params_ref::iterator it = u.params().erase( u.params().begin() );
assert( u.encoded_query() == "last=Doe" );
Linear in `this->url().encoded_query().size()`.
Throws nothing.