Update HttpHeaderName conversion operators to satisfy MSVC

MSVC produces errors other compilers do not. Is MSVC more correct? Unsure. Skip these tests in MSVC until we can figure out the more correct set of overloads for the conversion operators.

```
FAILED: [code=2] google/cloud/CMakeFiles/common_internal_http_header_test.dir/internal/http_header_test.cc.obj
sccache C:\PROGRA~1\MICROS~2\2022\ENTERP~1\VC\Tools\MSVC\1444~1.352\bin\Hostx64\x64\cl.exe /nologo /TP -DABSL_CONSUME_DLL -DGOOGLE_CLOUD_CPP_HAVE_GETRUSAGE=0 -DGOOGLE_CLOUD_CPP_HAVE_RUSAGE_THREAD=0 -DGTEST_LINKED_AS_SHARED_LIBRARY=1 -DNOMINMAX -DOPENTELEMETRY_ABI_VERSION_NO=1 -DOPENTELEMETRY_STL_VERSION=2017 -ID:\a\google-cloud-cpp\google-cloud-cpp -external:IC:\b\vcpkg_installed\x64-windows\include -external:IC:\b -external:W0 /DWIN32 /D_WINDOWS /EHsc /O2 /Ob2 /DNDEBUG -std:c++17 -MD /bigobj /experimental:external /external:W0 /external:anglebrackets /W3 /wd4996 /WX /showIncludes /Fogoogle\cloud\CMakeFiles\common_internal_http_header_test.dir\internal\http_header_test.cc.obj /Fdgoogle\cloud\CMakeFiles\common_internal_http_header_test.dir\ /FS -c D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc
D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc(35): error C2440: '': cannot convert from 'google::cloud::rest_internal::v3_6::HttpHeaderName' to 'std::string'
D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc(35): note: 'std::basic_string,std::allocator>::basic_string': ambiguous call to overloaded function
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\xstring(716): note: could be 'std::basic_string,std::allocator>::basic_string(const std::basic_string,std::allocator> &)'
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\xstring(1027): note: or 'std::basic_string,std::allocator>::basic_string(std::basic_string,std::allocator> &&) noexcept'
C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\xstring(768): note: or 'std::basic_string,std::allocator>::basic_string(const _Elem *const )'
with
[
_Elem=char
]
D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc(35): note: while trying to match the argument list '(google::cloud::rest_internal::v3_6::HttpHeaderName)'
D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc(35): error C3889: call to object of class type 'testing::internal::PredicateFormatterFromMatcher>': no matching call operator found
C:\b\vcpkg_installed\x64-windows\include\gmock/gmock-matchers.h(1644): note: could be 'testing::AssertionResult testing::internal::PredicateFormatterFromMatcher>::operator ()(const char *,const T &) const'
D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc(35): note: 'testing::AssertionResult testing::internal::PredicateFormatterFromMatcher>::operator ()(const char *,const T &) const': expects 2 arguments - 1 provided
D:\a\google-cloud-cpp\google-cloud-cpp\google\cloud\internal\http_header_test.cc(35): error C2737: 'gtest_ar': const object must be initialized

```

google/cloud/internal/http_header_test.cc、特に35行目で失敗しているアサーションから始め、そこから呼び出される HttpHeaderName の変換演算子を調べてください。MSVC で失敗を再現し、他のコンパイラとオーバーロードの動作を比較してください。テストが意図したカバレッジを弱めることなく MSVC でビルドに成功し、パスすれば完了です。

索引モデルが issue の本文から書いたものです。