Change in H+ ion concentration of water on boiling
It is an interesting question, however there is a misconception. You are giving too much attention to the word neutral. The word neutral is just a qualitative version. Numbers and equations are more accurate way to discuss science. 

You have to see how water's $\pu{pH}$ at 100 °C is calculated. How do you get those numbers? Lets us do it algebraically, so that the word neutral does not matter.

Autoionization of water at temperature T:$\ce{ H2O &lt;=&gt; H+  + OH-}$, if the conc. of $\ce{H+}$ is x, the quantity of $\ce{OH-}$ is also x, if you have accepted the ionic product of water is

$K_\mathrm{w} = [\ce{H+}][\ce{OH-}] = x^2=
51.3\times{10^{-14}}$ 

Solve for $x = [\ce{H+}] =&gt; -log [\ce{H+}] = 6.14$

So the numbers prove that $\pu{pH}$ has changed. Water has become slightly acidic. If you associate the word neutral = 7.0 at 25 °C. However, if you use the word neutral with respect to water temperature, then $\pu{pH}$ of 6.14 is neutral at 100 oC. Ultimately, the numbers Kw and pH matter, nobody will quibble about the word neutral in serious research.

Water is the most amazing solvent in Nature. People are still exploring its properties and it is yet to be fully understood. For example, I just heard in a conference that someone was trying to understand what happens on the surface of micron sized water droplets. "Pure" water showed very odd chemical effects which have never been shown before.

Read this well written Wikipedia article: https://en.wikipedia.org/wiki/PH#cite_note-2