Merge pull request #587 from ctiller/chex2

src/core/security/credentials.c

src/core/surface/channel.c

src/core/transport/chttp2_transport.c

src/core/transport/metadata.c

src/core/transport/metadata.h

test/core/channel/channel_stack_test.c

test/core/channel/metadata_buffer_test.c

test/core/security/credentials_test.c

test/core/transport/chttp2/hpack_parser_test.c

test/core/transport/chttp2/hpack_table_test.c

test/core/transport/chttp2/stream_encoder_test.c

test/core/transport/metadata_test.c

test/core/transport/transport_end2end_tests.c