Merge branch 'core_validate_metadata' into ruby_metadata_downcasing

BUILD

Makefile

binding.gyp

build.yaml

gRPC.podspec

grpc.gemspec

include/grpc/grpc.h

package.json

src/core/surface/call.c

src/core/surface/validate_metadata.c

src/core/transport/chttp2/bin_encoder.c

src/core/transport/chttp2/bin_encoder.h

src/core/transport/chttp2/hpack_encoder.c

src/core/transport/chttp2/hpack_parser.c

src/core/transport/metadata.c

src/core/transport/metadata.h

test/core/transport/chttp2/bin_encoder_test.c

tools/doxygen/Doxyfile.core.internal

tools/run_tests/run_interop_tests.py

tools/run_tests/sources_and_headers.json