[2.x] Add castAsJson() to Database

[phh]

Q	A
Bug fix?	no
New feature?	yes

If you want to work with assertDatabaseHas() and determine if some json exists, you need to cast it as json, using the $this->castsAsJson(). This adds the proxy method that already exists in InteractsWithDatabase.

Usage:

assertDatabaseHas('posts', [
    'data'   => castAsJson(['status' => 'publish'])
]);

[phh]

When trying to either use string, array or something like that you just get "wierd" errors like:

Failed asserting that a row in the table [posts] matches the attributes {
    "data": "{\"status\": \"publish\"}"
}.

Found: [
    {
        "data": "{\"status\": \"publish\"}"
    }
].

[phh]

Seems like a duplicate of #42