Merge pull request #1292 from jboeuf/load_file_with_null_term

src/core/security/google_default_credentials.c

src/core/security/security_context.c

src/core/support/file.c

src/core/support/file.h

test/core/security/create_jwt.c

test/core/security/fetch_oauth2.c

test/core/support/file_test.c