BUG #14138: Inconsistent rounding behavior in float4 coercion

The following bug has been logged on the website:

Bug reference: 14138
Logged by: Charles
Email address: cooper(dot)charles(dot)m(at)gmail(dot)com
PostgreSQL version: 9.5.3
Operating system: Linux 3.16.0.70-generic
Description:

Based on my reading of the documentation
(http://www.postgresql.org/docs/9.5/static/datatype-numeric.html) float4
only supports up to 6 decimal places of precision (even if the number fits
comfortably in a 4-byte float). This is fine but I am finding it difficult
to predict which way the float will truncate to:

```
elsendb=> select 15422.55 :: float4;
float4
---------
15422.5
(1 row)

elsendb=> select 11143.15 :: float4;
float4
---------
11143.2
(1 row)

```

Regardless of the default rounding mode (up / down / half even /
you-name-it) I would expect these two to round in the same direction -- or
at least the rounding algorithm to be specified and documented.

Platform information:
charles(at)futurelabs:~$ uname -r
3.16.0-70-generic

charles(at)futurelabs:~$ /lib/x86_64-linux-gnu/libc-2.19.so
GNU C Library (Ubuntu EGLIBC 2.19-0ubuntu6.7) stable release version 2.19,
by Roland McGrath et al.
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.8.2.
Compiled on a Linux 3.13.11 system on 2016-02-16.
Available extensions:
crypt add-on version 2.1 by Michael Glad and others
GNU Libidn by Simon Josefsson
Native POSIX Threads Library by Ulrich Drepper et al
BIND-8.2.3-T5B
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<https://bugs.launchpad.net/ubuntu/+source/eglibc/+bugs>.

Best,
Charles