Added the test to be robust,

which is that the build attribute is not treated as a legacy attribute.
Slightly refactored the MockPostprocessor class.
Supported using ICustomization type

for the constructor parameter 'fixtureType' of SpecAttribute.
Introduced the CreateCustomization method,

and improved or created the XML documentations.
Added the descriptive exception message.
Renamed the parameter to 'fixturingType'.
Rectified the failed tests.
Implemented the CreateCustomization method.
Introduced the new FixturingType property.
Deprecated the FixtureType property

and used the FixturingType property instead.
Added the test to be robust.
Addressed the code analysis warning.
Incremented minor version,

in order to indicate that the fixturing parameter of construtor of
SpecAttribute can accept the IFixture sub-type as well as
ICustomization. Also, this introduced the virtual the
CreateCustomization method in SpecAttribute to be able to override in a
sub-type of it.

The old FixtureType property was deprecated as not matched with the
fixturingType parameter.