Merge pull request gorilla#114 from shkw/fine-grained-decoding-error

Fine grained decoding error

Fix handling of empty value. (gorilla#104)

If a value is decoding into a type that implements the
encoding.TextUnmarshaler interface, the decoder should use it's
UnmarshalText method in all instances including an empty value.

Previously, would ignore the method decoding the field as the empty
value of the type.

Fix slice of structs TextUnmarshaler. (gorilla#103)

Fix handling of situation where a slice of structs implments the
encoding.TextUnmarshaler interface, previously it would return "invalid
path" error.

Includes, some minor refactoring and documentation to clarify
`isUnmarshaler` output.

Add go.mod file