fix ApplyRangeAttribute failing when min/max is specified as string

[sindrekroknes]

For cultures not using '.' as the decimal separator, JsonSchemaGenerator.ApplyRangeAttribute fails for Range attributes like this

[Range(typeof(decimal), "5.5", "10.5")]
public double Decimal { get; set; }

This PR fixes that by using InvariantCulture

[lahma]

LGTM, thank you!