Define copy constructor for instance.

Needed because the underlying CURLWrapper is not copied but freshly created, so
access_token, proxy, cainfo and useragent have to be set.

include/instance.hpp

@@ -64,7 +64,7 @@ public:
      *
      *  @since  0.5.2
      */
-    Instance(const Instance &other) = default;
+    Instance(const Instance &other);
 
     //! Move constructor
     Instance(Instance &&other) noexcept = delete;

src/instance.cpp

@@ -39,6 +39,21 @@ Instance::Instance(const string_view hostname, const string_view access_token)
     set_access_token(access_token);
 }
 
+Instance::Instance(const Instance &other)
+    : CURLWrapper{other}
+    , _hostname{other._hostname}
+    , _baseuri{other._baseuri}
+    , _access_token{other._access_token}
+    , _max_chars{other._max_chars}
+    , _proxy{other._proxy}
+    , _post_formats{other._post_formats}
+    , _cainfo{other._cainfo}
+    , _useragent{other._useragent}
+{
+    CURLWrapper::setup_connection_properties(_proxy, _access_token,
+                                             _cainfo, _useragent);
+}
+
 uint64_t Instance::get_max_chars() noexcept
 {
     constexpr uint64_t default_max_chars{500};