Error.Source should be an object not a string

[jaredcnance]

From the spec:

source: an object containing references to the source of the error, optionally including any of the following members:

• pointer: a JSON Pointer [RFC6901] to the associated entity in the request document [e.g. "/data" for a primary data object, or "/data/attributes/title" for a specific attribute].
• parameter: a string indicating which URI query parameter caused the error.

JsonApiDotNetCore/src/JsonApiDotNetCore/Internal/Error.cs

Line 62 in 9c54535

public string Source { get; set; }

[jaredcnance]

Closed by #369