moved-from container with unequal allocator will not be modified:

Affects:
- flat_buffer
- multi_buffer
- http::fields

fixes #1832
closes #1834

include/boost/beast/http/impl/fields.hpp

@@ -382,7 +382,6 @@ basic_fields(basic_fields&& other, Allocator const& alloc)
     if(this->get() != other.get())
     {
         copy_all(other);
-        other.clear_all();
     }
     else
     {
@@ -1139,7 +1138,6 @@ move_assign(basic_fields& other, std::false_type)
     if(this->get() != other.get())
     {
         copy_all(other);
-        other.clear_all();
     }
     else
     {