Query parameters and Headers should not be added to the generated request if they are undefined

[ghost]

In Servant the QueryParam adds its parameter as a Maybe type because query parameters and headers are optional. However, it seems like the vanilla JS generated code always appends the parameters even if undefined.

I think it's a good idea if the generated JS checks for undefined query parameters and headers and appropriately omits them from the URL.

[ghost]

It's probably a good idea to still send null values though, because we may want to tell the difference between null and undefined, in other words I might parse null as Just Nothing :: Maybe (Maybe a) to distinguish the two.

[ghost]

I will try to make a pull request...