Adding support for params without value.

[vuoriliikaluoma]

Adds support for params without value when set to null. (test included)
undefined can still be used to exclude the param.

Example:
This params object: { foo: null, bar: "baz", qux: undefined }
Becomes this query string: ?foo&bar=baz

This resolves #1139.

[vuoriliikaluoma]

Note: Mocha tests weren't working to start with. Tried npm run test before any changes.
I ran the karma tests with .\node_modules\.bin\grunt karma:single and they passed.

[nickstew]

Looks good to me. Minimal change. If not stated in the documentation, this should be expressly stated in the README. Should there be a flag to treat null and undefined as the same? I see each pattern being used in production environments.

[vuoriliikaluoma]

@nickstew as it is currently; we have full control over the resulting query:
{ key: undefined } to ? (not included)
{ key: null } to ?key
{ key: '' } to ?key=
{ key: 'null' } to ?key=null

[yasuf]

this is sort of a breaking change so I'll wait until 0.20.0 or 1.0.0 to release this, probably right after 0.19.1