Json columns are read as JSON::PullParser instead of JSON::Any

[matthewmcgarvey]

Fixes #125

Currently, if you have a json column and read it with this library, it is automatically turned into a JSON::Any. It makes sense when you don't care about the structure, but there's two problems with doing that:

1. JSON::Any is immutable so that values cannot be changed and then saved back to the database
2. You cannot convert a JSON::Any into a JSON::Serializable

This PR changes the default deserialization of json columns from JSON::Any to JSON::PullParser. The benefit we get from this is that it can easily be converted into a JSON::Any or a JSON::Serializable. It is still not mutable but it's easier to get to a mutable setting:

properties = result_set.read(JSON::PullParser)
mutable_properties = Hash(String, String).new(properties)

I added a method specifically for still supporting calling result_set.read(JSON::Any) with hopes that will cause the least amount of breakage, because this is definitely a breaking change.

[bcardiff]

LGTM. I feel we need a read_io directly that will allow us to use the stream directly into de pullparser or any other consumer of IOs. I'm mentioning this to share this idea that should be done in crystal-db at some point I think.

[will]

Thanks! Could you please add a line to the changelog file about this?

[will]

Awesome, thanks!