[codegen] InlineModelResolver doesn't copy `required` parameter to new reference property

Description

https://github.com/swagger-api/swagger-codegen/blob/v2.2.1/modules/swagger-codegen/src/main/java/io/swagger/codegen/InlineModelResolver.java#L249
Here new reference property is created instead on inline object property. But flag required is not copied to newly created property definition.
It leads that generated model is not relevant to swagger description.

Also in ModelImpl - https://github.com/swagger-api/swagger-core/blob/v1.5.10/modules/swagger-models/src/main/java/io/swagger/models/ModelImpl.java#L176 the list of required fields is calculated according to properties, not appropriate variable - https://github.com/swagger-api/swagger-core/blob/v1.5.10/modules/swagger-models/src/main/java/io/swagger/models/ModelImpl.java#L22
It looks like data duplication (list and value in properties), I would remove the list.

Swagger-codegen version

2.2.1 and current master

Start with InlineModelResolver.java at the referenced line to trace creation of the reference property, then inspect ModelImpl.java at the referenced lines to compare required-field storage and calculation. Reproduce the inline-object case with a required property and verify that the generated model remains consistent with the Swagger description, including the required-field handling.