Remove sharp edge (uninitialised member m_filter_type) when using the compiler-generated constructor for BlockFilter.
Before (but after added test):
$ src/test/test_bitcoin -t blockfilter_tests/blockfilter_basic_test
Running 1 test case...
test/blockfilter_tests.cpp(118): error: in "blockfilter_tests/blockfilter_basic_test": check default_ctor_block_filter_1.GetFilterType() == default_ctor_block_filter_2.GetFilterType() has failed [ != ]
*** 1 failure is detected in the test module "Bitcoin Test Suite"
After:
$ src/test/test_bitcoin -t blockfilter_tests/blockfilter_basic_test
Running 1 test case...
*** No errors detected