fix(vue-app): add leading slash for payload hydration

[danielroe]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)

Description

Fixes regression reported https://www.github.com/nuxt/nuxt.js/issues/8107#issuecomment-776227825 and https://www.github.com/nuxt/nuxt.js/issues/8107#issuecomment-776222703.

Checklist:

• I have added tests to cover my changes
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #8802 (5d1a5fd) into dev (c221f60) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #8802   +/-   ##
=======================================
  Coverage   66.21%   66.21%           
=======================================
  Files          93       93           
  Lines        4026     4026           
  Branches     1102     1102           
=======================================
  Hits         2666     2666           
  Misses       1099     1099           
  Partials      261      261           

Flag	Coverage Δ
unittests	66.21% <ø> (ø)

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