avoid frame breaks between block title and body for rectangular blocks

Test file:
```
\documentclass{beamer}

\usecolortheme{orchid}

\begin{document}

\begin{frame}[allowframebreaks]
  \begin{block}{title}
  \vspace{5cm}
  \end{block}
  \begin{block}{title}
   \vspace{5cm}
  \end{block}
\end{frame}

\end{document}
```

CHANGELOG.md

@@ -13,6 +13,7 @@ a major and minor version only.
 - unified usage of `(sub)section in head/foot` template in headlines
 - simplify decision tree for `\beamer@howtotreatframe` (see #874)
 - added `\maketitle`, `\title`, `\author`, `\institute` and `\date` to the exceptions for `ignorenonframetext`
+- avoid frame breaks between block title and body for rectangular blocks
 
 ### Fixed
 

base/themes/inner/beamerinnerthemedefault.sty

@@ -392,7 +392,7 @@
   \par\vskip\medskipamount%
   \begin{beamercolorbox}[colsep*=.75ex]{block title}
     \usebeamerfont*{block title}\insertblocktitle%
-  \end{beamercolorbox}%
+  \end{beamercolorbox}\noframebreak%
   {\parskip0pt\par}%
   \ifbeamercolorempty[bg]{block title}
   {}
@@ -409,7 +409,7 @@
   \par\vskip\medskipamount%
   \begin{beamercolorbox}[colsep*=.75ex]{block title alerted}
     \usebeamerfont*{block title alerted}\insertblocktitle%
-  \end{beamercolorbox}%
+  \end{beamercolorbox}\noframebreak%
   {\parskip0pt\par}%
   \ifbeamercolorempty[bg]{block title alerted}
   {}
@@ -428,7 +428,7 @@
   \par\vskip\medskipamount%
   \begin{beamercolorbox}[colsep*=.75ex]{block title example}
     \usebeamerfont*{block title example}\insertblocktitle%
-  \end{beamercolorbox}%
+  \end{beamercolorbox}\noframebreak%
   {\parskip0pt\par}%
   \ifbeamercolorempty[bg]{block title example}
   {}