add support for flag type params with no value
add support for flag type params with no value

also adding additional test cases to ensure correct behavior.

get_param("reverse") does not return Some("") as expected for query string ?reverse
The expected behavior for URLParams is to treat params with no = in them as if the same as an empty value
localhost:80/api/users?reverse should be equivalent to localhost:80/api/users?reverse=

for reference:
https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams#empty_value_vs._no_value

tomaka/rouille