ft: improve handling of shiny and htmltools tags

[kennedymwavu]

closes #76 & #77

• remove redundant calls to convert_body() in the Response methods: send(), sendf() and text()
• refactor convert_body() by removing unnecessary checks
• improve handling of shiny & htmltools tags

with this PR, this reprex now works as expected:

# app.R
library(ambiorix)
library(htmltools)

app <- Ambiorix$new()

app$get("/", \(req, res){
  x <- tagList(
    tags$h1("hello"),
    tags$h2("there")
  )
  res$send(x)
})

app$start()

[JohnCoene]

Looks good, thanks!