Fix basic_streambuf::capacity

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * Change implicit to default value in example
 * Tidy up some declarations
+* Fix basic_streambuf::capacity
 
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include/beast/core/impl/basic_streambuf.ipp

@@ -533,7 +533,7 @@ basic_streambuf<Allocator>::capacity() const
 {
     auto pos = out_;
     if(pos == list_.end())
-        return 0;
+        return in_size_;
     auto n = pos->size() - out_pos_;
     while(++pos != list_.end())
         n += pos->size();