"Parameters" class lacks a method to add a parameter

[mexus]

While writing an application which uses the cpr library I found impossible to add parameters to a request in a dynamic fashion.
For example, I have some parameters backed up in a std container, and I want to add all of them to the cpr::Parameters object, but I don't know the size of the container in advance.
Maybe I'm misleading something, maybe this funcionality already exists in the project, nevertheless I wasn't able to find it. Please correct me if I am wrong, but I think this simple patch can be useful.

[whoshuu]

Hey @mexus, this is a pretty good find. You're right, there's no way to change Parameters after its been instantiated, except through direct modification of the content member. If possible, can you add a test after this one that demonstrates the expected behavior?

[mexus]

@whoshuu , sure thing!

[whoshuu]

Thanks @mexus, merged!

[mexus]

@whoshuu, wow, that's fast =) BTW, thank you for your nice lib! Finally we have some sane and true-c++ curl wrapper!