do_set_request() returns wrong reply with (in)valid specifier

[yadiw]

In unit tests for dripline.core.Endpoint, for its method on_set_request():

Given a request with a valid specifier:

• expected: the value of the existing attribute is updated, a reply with return code 0 is returned
• actual: the value of the existing attribute is not updated, a reply with return code 201 is returned

Given a request with an invalid specifier:

• expected: a reply with return code 201 is returned
• actual: a new attribute with given value is added to the endpoint, a reply with return code 0 is returned

p.s. if we run these tests for more than once, an segmentation fault would occur, and this is discussed in another issue.

[laroque]

In the tests, the methods now handle the ThrowReply producing those errors, this is resolved on branch dl3/unittest-fixes (No PR yet because it requires driplineorg/dripline-cpp:develop)

[laroque]

done last friday