dump/source problem with hclust object (PR#4361)

For the record, this is due to a storage mode problem: the merge component
gets restored as double.  I've modified plot.hclust to detect that.

    library(mva)
    data(USArrests)
    hc <- hclust(dist(USArrests), "ave")

    plot(hc)              # OK

    dump(c("hc"), "tst")
    rm(hc)
    source("tst")

    plot(hc)              # Error in plot.hclust(hc) : invalid dendrogram input

The same problem occurs with dput/dget

--please do not edit the information below--

Version:
 platform = i686-pc-linux-gnu
 arch = i686
 os = linux-gnu
 system = i686, linux-gnu
 status =
 major = 1
 minor = 7.1
 year = 2003
 month = 06
 day = 16
 language = R

Search Path:
 .GlobalEnv, package:methods, package:ctest, package:mva,
package:modreg, package:nls, package:ts, Autoloads, package:base

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Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595