Improve error messages for Be validation

[ronaldkroon]

The Be and BeEquivalentTo validations were very similar. Kept the BeEquivalentTo and removed the Be, because the first one has a better name; it shouldn't be the same object, but an equivalent object. [BREAKING CHANGE] The error message now specifies the path in the JSON and the type of the first found difference. Also added an overload that accepts and parses a string representation of a JSON document. Without the overload a string would implicitly be converted to a JValue and compared as such, which is rarely what you want. To use that behavior an explicit JValue must be created. [BREAKING CHANGE]

[ronaldkroon]

@dennisdoomen do you have time to look at this?

[ronaldkroon]

Agreed with @dennisdoomen to:

• rename Be to BeEquivalent
• add overload that takes a string and parses that to JSON

[ronaldkroon]

Changed as per agreement, please have a look, @dennisdoomen.