CoercionConfigs (2.12): make existing coercions configurable

[cowtowncoder]

Now that #2113 -- core system for configuring allowed coercions -- has been implemented, what is needed next is to change actual deserializers to make use of configuration. This mostly covers existing implicitly allowed coercions (or possibly, blocked). So far existing changes cover:

• Allowing coercion from empty String (and in case of XML, blank) into POJOs, Collections/arrays, Maps -- and possibly many other types (if they extend StdDeserializer, or deserializers that extend it).
• Allowing String coercion into primitives (mostly numbers, but also booleans/Booleans).

but most other things are not yet checked.

I created this placeholder issue to allow users to comment; I will likely create separate issues for individual deserializer changes (or some grouping of those).

[cowtowncoder]

Just to seed discussion, here are some thoughts:

1. Boolean type: allowing (or not) use of integer number values (as of now, coercion from 0 is false, non-0 true) -- and (I have to check this), verify if floating-point numbers are blocked or not
2. JDK string-like types: verify existing EmptyString coercion
3. Date/time types (Joda, Java 8); at least check EmptyString handling. Probably same for floating-point numbers, and (in case Array type is not standard serialization), arrays.
4. Guava: verify that Collection, Map follow EmptyString coercion

Some existing tickets that seem relevant too:

• Allow case insensitive deserialization of String value into boolean/Boolean (esp for Excel) #1852 (case-insensitive booleans) -- not sure how this can be addressed
• --2066 (UUID from empty String)--

Finally, I added coercion-config label to use for these issues.

[cowtowncoder]

Implemented, at least:

• All existing coercions to boolean/Boolean (from String, integer number) converted to use new mechanism (but old ones also work)
• Float-to-integer checks similarly upgraded
• Coercion from empty String to Map verified
• From-empty-String coercions for JDK scalar types verified

This leaves

• Java 8 date/time
• Collection types (Guava esp)
• Possible additional leniency settings for boolean

[cowtowncoder]

Implemented "Empty String" coercion checks for Joda and Java 8 date-time types.

Will need to add one new coercion config check wrt Enums but will file separate issue for that.