[csharp] map number/no format to Decimal

[jimschubert]

Changes the default primitive for number with no specified format from double to decimal in C#.

Other number/integer mappings remain unchanged:

type	format	C# type
number	float	System.Float
number	double	System.Double
number	------	System.Decimal
integer	int64	System.Int64
integer	int32	System.Int32
integer	------	System.Int32

See #2967 and #2813.

@wing328 this seems like an extremely simple fix, and I feel like I've missed something. I generated the csharp client and verified that types looked fine and tests passed.

[wing328]

@jimschubert I agree with you that this should be a simple fix. I'll mark this as a breaking change with fallback as number will be mapped to decimal instead of double.

[wing328]

Upgrade note from 2.1.6 to 2.2.0

Starting from 2.2.0, number (without format) will be mapped to decimal? for C#-related generator to make it more consistent with other languages (e.g. Java).

To fallback, please specify the format of the property/parameter as double

Ref: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#data-types