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

Affects:
- flat_buffer
- multi_buffer
- http::fields

fixes #1832
closes #1834

include/boost/beast/core/impl/flat_buffer.hpp

@@ -124,8 +124,6 @@ basic_flat_buffer(
         end_ = nullptr;
         max_ = other.max_;
         copy_from(other);
-        other.clear();
-        other.shrink_to_fit();
         return;
     }
 
@@ -436,8 +434,6 @@ move_assign(basic_flat_buffer& other, std::false_type)
     if(this->get() != other.get())
     {
         copy_from(other);
-        other.clear();
-        other.shrink_to_fit();
     }
     else
     {

include/boost/beast/core/impl/multi_buffer.hpp

@@ -502,8 +502,6 @@ basic_multi_buffer(
     {
         out_ = list_.end();
         copy_from(other);
-        other.clear();
-        other.shrink_to_fit();
         return;
     }
 
@@ -1060,8 +1058,6 @@ move_assign(basic_multi_buffer& other, std::false_type)
     if(this->get() != other.get())
     {
         copy_from(other);
-        other.clear();
-        other.shrink_to_fit();
     }
     else
     {