Add a convenience API for building a query string

[rynowak]

Routing currently has it's own code for building a query string from key-value pairs. It would be great to have an API like this in http abstractions so we don't end up repeating ourselves.

See: https://github.com/aspnet/Routing/blob/dev/src/Microsoft.AspNet.Routing/Template/TemplateBinder.cs#L221

[lilith]

Ah, but with which rules? Serialization is far simpler that parsing, but it still needs to be clearly defined.

[glennc]

In Microsoft.AspNet.WebUtils, once we know what it does :)

[yishaigalatzer]

any updates?

[glennc]

@Tratcher is going to work on it this week.