Replace using namespace with namespace.

src/exceptions.cpp

@@ -18,10 +18,12 @@
 
 #include <utility>
 
-using namespace mastorss;
 using std::to_string;
 using std::move;
 
+namespace mastorss
+{
+
 HTTPException::HTTPException(const int error)
     : error_code{static_cast<uint16_t>(error)}
 {}
@@ -61,3 +63,4 @@ const char *FileException::what() const noexcept
 {
     return _message.c_str();
 }
+} // namespace mastorss